As is well known, emissions control of flue gases is one of the more challenging problems in preventing environmental deterioration, and a wide range of efforts have been made toward decreasing harmful emissions in flue gases, particularly, nitrogen oxides, sulfur oxides and carbon dioxide. These emissions are particularly troublesome in connection with oil or coal-fired combustion furnaces, e.g. those furnaces used by power plants for generating electricity, heating plants for generating a sensible heat for building control, and power plants for powering industrial processes and factories. Most of the effort in the art has been toward that of wet or dry scrubbing the flue gases to remove, essentially, the acid-causing compounds contained in the flue gases, in order to mitigate consequential formation of acid rain. Some of these scrubbing processes have created new waste disposal problems, in and of themselves, and beyond that, those scrubbing processes are relatively ineffective in simultaneously removing other common pollutants from such flue gases, such as nitrogen and sulfur oxides.
Further, of increasing concern is the large amount of carbon dioxide contained in such flue gases, since evidence is now being generated that increased quantities of carbon dioxide expelled into the atmosphere with such flue gases contribute to the perceived global warming. Conventional processes, e.g. scrubbing processes, are not capable of removing substantial quantities of carbon dioxide, in view of the low solubility thereof in most scrubbing materials, and, therefore, typical flue gas emissions, at present, contain large amounts of carbon dioxide.
Usual scrubbing processes are also relatively inefficient in removing nitrogen dioxide and sulfur dioxide, as well as other pollutants such as unburned fuel and carbon monoxide. As a result, the scrubbed flue gases still contain considerable amounts of such pollutants, which not only continue to degrade the environment, but also necessitate the use of so-called "high" stacks for dispersal of the flue gases into the atmosphere. In this latter regard, a so-called "high" stack is required such that the exit of the still polluted flue gases from the stack is sufficiently high above ground that under usual atmospheric conditions, those flue gases will move upwardly into the atmosphere and away from populated areas near the stack. This is opposed to the so-called "low" stacks, which are much shorter than the "high" stacks, which "low" stacks are appropriate only for emissions of gases with quite low levels of pollutants, particularly where those pollutants are of such low level that they may be dispersed near the ground in populated areas without causing harm to either the population or the environment. Those high stacks, in turn, require considerable buoyancy of the flue gases, in order that the flue gases will move upwardly in the "high" stacks and be dispersed upwardly into the atmosphere. That buoyancy is obtained by keeping the flue gases in the stacks relatively hot, and, therefore, that heat energy in the flue gases cannot be extracted for other purposes.
In view of these considerable deficiencies in the usual scrubbing-type means for removing pollutants, the art has made a considerable effort to provide improved processes and apparatus for removing gaseous pollutants from polluted flue gas streams. These efforts have ranged widely in technical principle. Thus, for example, U.S. Pat. No. 4,988,490 suggests an absorptive process, especially that of using pressure swing absorption for removal of pollutants.
It is well known that the gaseous pollutants in flue gas can be condensed or frozen with sufficient cooling and pressure, of the flue gas, and the condensed or frozen gaseous pollutants can, therefore, be separated from the flue gas. In U.S. Pat. No. 4,513,573, a process is described where flue gas from a steam generating power plant is separated into condensable and non-condensable components by freezing the flue gas. In a specific embodiment of that patent, the combustion chamber of the power plant is maintained under sufficiently high pressure so as to avoid the need for additional downstream compression of the flue gas to cause condensation. The frozen components are recovered and, through sublimation and vaporization, are processed for reuse in other industries. The use of a high pressure combustion chamber, however, causes considerable technical problems and cannot be widely adopted.
A little different approach from that of the foregoing patent is described in U.S. Pat. No. 5,205,843, where the gas stream is treated for removal of pollutants using a combination of membrane separation and condensation, and a similar approach is also described in U.S. Pat. No. 5,089,033.
These processes which depend, at least in part, on condensation or freezing of the pollutants by lowering the temperature of the flue gas, are generally referred to in the art as cryogenic separation processes, although that term is not meant to imply a specific minimum reduced temperature, but simply the cooling of the flue gas until condensation or freezing of the separate pollutants occur, which cooling may or may not be with pressurization of the flue gas.
The difficulty with such cryogenic separation is also well known in the art. Thus, the pressure of flue gas from the usual furnace is very low and, therefore, does not lend itself to separating pollutants by condensation without significant compression and without considerably lowering the temperature of the flue gas. Because of the large flow of flue gas from power plants and other combustion process plants, the required compression horse power, alone, can be quite high. In U.S. Department of Energy Report No. ER-30194, a Research Needs Assessment on Carbon Dioxide from Fossil Fired Power Plants, estimates an energy penalty of 55% to remove carbon dioxide by cryogenic means from coal-fired power plants. This estimate, however, does not account for the energy penalty in concomitant cooling of the flue gas or the removal of nitrogen oxides and sulfur oxides.
In this latter regard, the concentration of components of flue gas, such as nitrogen oxides and sulfur oxides, is very low and, hence, their partial pressures are very low, requiring considerable energy in the form of cooling or pressure for adequate condensation thereof. Thus, cryogenic separation of pollutants from flue gases would seem to be a very unattractive approach for removal of pollutants from those flue gases, in view of the serious energy penalty involved.
It would, therefore, be of considerable advantage to the art to provide method and apparatus for removal of gaseous pollutants from a polluted flue gas stream from an oil or coal-fired combustion furnace by cryogenic means, which is capable of very substantially reducing the amount of pollutants in the flue gas stream, as opposed to the more conventional approaches in the art, as explained above, but where the cryogenic means does not engender the very serious energy penalty discussed above.